Sheathed resistance or electric heaters have been widely used in many fields because of their very excellent performance, quality and convenience, extending the commercial range including not only domestic electric articles, but also specific applications such as in various industries, space developments, atomic power services and the like. Among various classes of sheathed resistance heaters, sheathed heaters for high temperature purposes are considered to have more and more increasing applications in the future.
Upon reviewing the performance and quality of sheathed resistance heaters in a world-wide sense, they have drawbacks in that the insulation resistance of the heaters in a working condition (hereinafter referred to as insulation resistance under self-heating conditions) lowers as a function of time, coupled with another disadvantage that it takes only a short time before disconnection of the heating wire.